Web coating method

ABSTRACT

In a coating method, the web is conveyed and supported on a rotating roll. Liquid coating material is applied by atomizing the material with air to create a mist adjacent to and contiguous with the supported web. A thin line of pressurized air from the discharge orifice of an air knife is directed toward the web on an angle against the direction of web travel and into a confined space away from the orifice to create a turbulent zone adjacent the web. Liquid coating material is metered under pressure into the turbulent zone through a slot of a chamber. The coating on the web is leveled by the same air stream in a second zone closely adjacent the air orifice.

United States Patent [1 1 Damrau Oct. 28, 1975 [54] WEB COATING METHOD[75] Inventor: Wayne A. Damrau, Wisconsin Rapids, Wis.

[22] Filed: Oct. 25, 1973 [21] Appl. No.: 409,403

Related US. Application Data [62] Division of Ser. No. 220,895, Jan. 26,1972, Pat, No.

[52] US. Cl. 428/537; 427/248; 427/345; 427/348; 427/424 [51] Int. Cl.B44D 1/44; B05C 1l/10; C23C 1l/00;BO5B 13/02 [58] Field of Search 117/64R, 102 R, 105.3,

[56] References Cited UNITED STATES PATENTS 2,135,406 1l/l938 MacDonald117/102 X 2,332,385 10/1943 Lauring 91/30 2,995,469 8/1961 Le Claire117/34 3,229,447 l/1966 Kosta ll7/102 X 3,297,472 l/l967 Gilman et al.l17/65.2 X 3,341,351 9/1967 Brewer 117/64 R 3,635,192 l/l972 Herzhoff eta1 118/63 3,678,890 7/1972 Ehrensing et al. 118/63 PrimaryExaminer-Michael R. Lusignan Attorney, Agent, or Firm-Gary, Juettner,Pigott & Cullinan 57 ABSTRACT In a coating method, the web is conveyedand supported on a rotating roll, Liquid coating material is applied byatomizing the material with air to create a mist adjacent to andcontiguous with the supported web. A thin line of pressurized air fromthe discharge orifice of an air knife is directed toward the web on anangle against the direction of web travel and into a confined space awayfrom the orifice to create a turbulent zone adjacent the web. Liquidcoating material is metered under pressure into the turbulent zonethrough a slot of a chamber. The coating on the web is leveled by thesame air stream in a second zone closely adjacent the air orifice.

7 Claims, 1 Drawing Figure US. Patent Oct. 28, 1975 1. WEB COATINGMETHOD CROSS REFERENCE This is a divisional application of my copendingapplication, Ser. No. 220,895, filed Jan. 26, 1972, now US. Pat. No.3,799,l l l, issued Mar. 26, I974..

BACKGROUND OF THE INVENTION This invention relates to a method forcoating webs and more particularly to a method for applying a thin coatof a coating composition to a moving flexible web composed of paper orthe like.

It has been customary heretofore to utilize so-called dip or applicatorroll devices with or without air or mechanical doctor blades to applyand spread a liquid composition onto one or both of the surfaces of amoving strip material or web. Coating compositions have also beenapplied to a moving web by pumping the composition through a slot in ahopper and onto the web surface. These methods have been satisfactory tothe extent that uniform and usable coatings have been achieved at normalcoat weights. At lower coat weights, such as thosein the order of about0.5 to 1 pound of solids per ream, it becomes increasingly difficult tomaintain uniformity by these methods, and many proposals to applyuniform low weight coatings have been made. A particular difficulty inapplying light weight coatings with blade coaters is the possibility ofexposingprominent surface fibers. Attempts at applying coatings byspraying have been generally unsuccessful because; of the necessity ofoverlapping the spray patterns, resulting in an uneven coating.

BRIEF DESCRIPTION OF THE INVENTION The coating device of the presentinvention comprises a low. pressure liquid applicator in-cooperationwith a high velocity air knife, both located closely adjacent oneanother and directed against the web along the width thereof. The airknife orifice is directed on an angle against the direction of webtravel in such a manner as to cause a thin stream. of airitoconverge-and impinge the web along a distinct zone immediately adjacentthe orifice. The air is deflected and becomes divergent and increasinglyturbulent toward the area of application after the first incidence ofimpingement and is confined adjacent the web by a spacer between the airknife and the outlet of the applicator. The liquid coating compositionis introduced-from a slot in the applicator toward the web and into theturbulent air from the air knife, such that the'composition is at leastpartially atomized. A mist of coatingmaterial is thus deposited on theweb, in contradistinction to direct liquid application of many prior artmethods. After deposition of the coating mist, the. coated web passesthrough the zone of convergence of the air knife whereby the wet coatingon theweb is leveled or skimmed.

While the method of the present invention is useful in applying a widerange of coating weights to a web,

the primary advantages are two in number, namely the ability to depositthin or light weight coatings and-the ability to generate a relativelycoarse or rough surface end a conventional coater may be placed inseries with the present apparatus for the purpose of applying coating ofhigh solids content, followed by a thin finish overcoat from thepresently described apparatus.

Another application may be in the manufacture of electrostatic paperswhere a relatively rough top coat on the sheet is desirable to give thesheet good tooth that is receptive to writing instruments.

Other advantages and objects of the present invention will becomeapparent from the following description of a preferred embodimentthereof, taken in connection with the appended claims and accompanyingdrawing.

' BRIEF DESCRIPTION OF THE DRAWING The FIGURE illustrated in the drawingis a vertical sectional view of the novel web coating apparatus, saidapparatus being shown in connection with a web support upon a-roll.

I DESCRIPTION OF THE PREFERRED EMBODIMENT In the preferred embodiment,the web coating apparatus, shown generally at 10, is supported in aposition to operate in conjunction with a web 12 of paper or the likepartially wrapped around and carried upon a backing roll 14. The backingroll 14 rotates about its axis in the direction indicated by the arrowto continuously move the web 12 in the same direction past the coatingapparatus and to enable application of a coating to the web in acontinuous fashion. The coating device is shown in isolation with thebacking roll, but it will be understood that the device may beincorporated directly in the line of a web forming machine, such as apaper machine, or may be employed in a separate coating and finishingline. Also, it will be understood that related conventional equipment,such as driers and calendering rolls, have been omitted from the presentdescription as being unessential for a full and complete understandingof the present invention.

The coating device generallycomprises an air impingement component orair knife 16 which cooperates with a liquid metering component 18located ahead of the air impingement component in terms of direction ofweb travel. The components 16 and 18 are pivotally mounted as anassembly and-are separated by a spacer 20. The assembly is substantiallyenclosed at its sides by parallel side walls, such as 22 which alsoencompass the sides of the backing roll in the vicinity of application.The air outlet orifice 24 of the air knife 16 is directed inward towardthe supported web and toward the outlet orifi'ce 26 of the meteringcomponent 18, and there is a substantially enclosed space between saidorifices defined by the spacer 20, side walls 22 and web 12, into whichair from the air knife is channeled and becomes turbulent. The turbulentair then impinges on liquid issuing from the orifice 26 of the meteringcomponent 18 and causes atomization of the coating liquid in an areaadjacent the moving web ahead of said orifice. The air knife 16 alsoserves the second function of skimming the wet coating on the web in avicinity immediately ad- (on a fine scale) when formulations containingsubstantial amounts of pigment are applied. For example, the

method may be employed in the application ofthin jacent to its outletorifice 24.

The air impingement component or air knife 16 may be adjustablysupported and more particularly comprises a partial cylindrical member28 defining a pienum chamber to which is connected at source ofpressurized pneumatic fluid (not shown), such as an air pressure tanksupplied by a pump. A pair of elongated converging lips 30 and 32 extendradially from the open side of the cylindrical member 28 to define thenarrow outlet orifice 24 that extends across substantially the entirewidth of the web 12 along a zone perpindicular to the sides of said web.One of the lips 32 may include a separate and replaceable end section34, which may comprise a flat outer surface 36 and a convexly curvedinterior surface 37, terminating in a sharp edge and partially definingthe air flow characteristics from the outlet orifice 24. The other lip30 comprises a flat interior surface 38, with the outer surface having atapered portion 42 defining a sharp edge.

The lips 30 and 32 are narrowly spaced from one another at the outlet 24and are closely spaced from the supported web 12 to cause a thin jet ofair to be directed toward the web surface. In order to define optimumair flow characteristics, the distance between the lips 30 and 38 at theoutlet 24 is in the order of about to thousandths of an inch. The angle41 of the air knife downward from horizontal toward the web ispreferably maintained from about 45 to 53, and optimum results areachieved at about 47. Also, the distance between the air orifice 24 andthe web, indicated at 43, is important in defining proper air flowcharacteristics and should be maintained between about oneeighth to onesixty-fourth inches. In this manner, the air issuing from the orifice 24will initially be convergent and will impinge the web 12 along a sharpline to achieve a flexible skimming effect.

The external surface 36 of the lip section 34 is connected to a flatsurface of the spacer 20, which may generally comprise a rectangular barextending across the width of the web. Preferably, the spacer 20 isadjustable relative to the air knife in a direction toward and away fromthe web, and the surface thereof facing the web and spaced therefrom mayhave a groove 45 therein. The groove 45 is preferably most sharplyrecessed immediately below the air outlet 24 to define an edge 44 evenwith the edge of the lip section 34. The part of the groove 42 adjacentthe liquid orifice 26 is more gradually curved to form a relativelythick edge 46, which projects slightly beyond the other edge 44.

The confined space between the groove 45 and the support web 12 forms achannel of fluid communication between the air orifice 24 and the liquidorifice 26 wherein the air stream, after its initial impingement uponthe web, may become divergent or turbulent before encountering thecoating liquid. In order to prevent excessive accumulation of thecoating material on the lips 30 and 32 of the air knife 16, it has beenfound that a thin, open end slot 48 between the lip section 38 and thespacer 20 facing the web will minimize this undesirable result. The slot48 is preferably in the order of about from one thirty-second toone-half inches in thickness and apparently causes minor air turbulencein a vicinity immediately below the air orifice 24, thereby minimizingspattering of liquid against the lip surfaces.

The liquid metering component 18 is connected to the side of the spacer20 opposite to that of the air knife 16 and comprises a flanged L-shapedchannel 50 secured from the spacer and carrying a corresponding reverseL-shaped blade support 52. The channel 50 and the support 52 togetherdefine a fluid tight chamber 53 for containing a liquid coatingcomposition, such as an aqueous mixture of a powdered mineral materialand an adhesive, which may comprise, for example, clay and starch. Aninlet 54 to the chamber 53 may be provided in the channel 50 throughwhich the desired coating mixture may be delivered under pressure from apump (not shown). A wide range of solids may be incorporated into thecoating material, such as between about 5 percent (or lower) to 37percent, and at viscosities ranging from at least 10 to 264 centipoisesat 20 rpm Brookfield.

A blade 56 having a tapered end is mounted on the support 52 and extendsacross the width of the web 12 in a plane substantially parallel to aplane tangential to the backing roll 14. The tapered end of the blade 56is preferably located in a position level with or behind the edge 46 ofthe spacer 20 at a distance of up to about three-sixteenths of an inch,said distance being indicated at 58. To achieve optimum results, theoffset spacing indicated at 58 is preferably adjusted to aboutone-eighth of an inch. The blade 56 is also adjustably spaced from thespacer 20 to define the liquid orifice 26, said orifice being separatedfrom the web by from three-sixteenths to three-eighths of an inch.

The metering component operates by means of maintaining positivepressure upon the liquid coating mixture within the chamber wherebyliquid is forced out of the orifice 26 toward the facing surface of theweb 12. In order to assure a uniform coating, the pressure within thechamber is maintained at a constant level within the range of about 0.1to 3 pounds per square inch. The liquid coating mixture is thereforeinjected into the turbulent atmosphere created by the air knife 16,which causes the coating to atomize and be deposited upon the web.

in the operation of the present apparatus, an excess of coating liquidis introduced through the orifice 26 and means are provided to collectand recirculate unused material back into the chamber 53. A barrier 60comprising a knife with a tapered edge extending closely adjacent thewidth of the web is spaced from and substantially parallel to the blade56, and is located ahead of the metering component 18 in terms of webtravel. The barrier 60 is also encompassed by the side walls 22 andprevents escape of atomized coating material, part of which is depositedon the web between the blade 56 and the barrier 60. A collecting chamberor separator pan and pump (not shown) may be located beneath theenclosed space between the blade 56 and the barrier 60, in order toreturn unused coating material to the chamber 53.

The operation of the coating apparatus and the method of the presentinvention will now be apparent. The coating apparatus is preferablysupported in the position shown in the drawing, that is, the air knifeis slanted downward about 47 from horizontal, with its orifice locatedbelow a horizontal plane passing through the axis of the backing roll14. lt will be obvious, however, in view of the atomization concept bywhich the apparatus operates, that the apparatus may be located in otheraxial positions with respect to the backing roll 14.

In the operation, the air knife 16 is pressurized to create anon-turbulent zone of air impingement immediately adjacent the orifice24 thereof and substantially turbulent zone adjacent the liquid orifice26. Thereafter, a stream of liquid coating material is injected Ithrough the orifice 26 into the turbulent air and not directly onto theweb 12. In this manner, the liquid is initially atomized to create aturbulent and homogeneous mist contiguous with the moving web, whereby afilm of substantially uniform and constant thickness is deposited on theweb between the orifice 26 and the barrier 60. The thus coated web isthen immediately passed through the non-turbulent zone of the air knife16, which serves to level or skim the coating without scraping coatingmaterial off of prominent surface fibers in the web. The web is thendried in a conventional drier and may then be subjected to a calenderingoperation.

The apparatus herein described is extremely versatile in terms ofadjustability and may apply coat weights ranging from 0.5 to 5 poundsper ream and at web speeds in excess of 1500 feet per minute. The coatweight may be varied by changing the pressure in the air impingementcomponent 16 or in the liquid chamber 53, by changing either or both ofthe orifice gaps 24 and 26, by adjusting the air knife angle 41, byincreasing or decreasing the distance between the apparatus and the roll14, or by varying the coating solids, or by combinations of any of theabove.

Another important feature of the present invention is that no parts ofthe coating apparatus contact the web. The coating is applied bydeposition of a mist of the liquid material onto the moving web, and thecoating is immediately leveled or skimmed by an air knife which is alsospaced from the web. The cooperation between the air impingementcomponent and the metering component is particularly advantageous inthat the air component serves the two-fold purpose of atomizing thecoating material and skimming the coated web. Also, the close proximityof the air knife to the metering device allows less time for moisture inthe coating to migrate into the web and also allows for convenientrecirculation of the unused coating liquid.

Other important advantages of the presently described coating device arethe ability to apply coatings of low solids content and to coverprominent surface fibers in the web. Since the apparatus does notcontact the web, it may be used to apply low coat weights onto adilutant wet coated surface without unduly disrupting the previous coat,and without substantial increase in the drying load. For example, theprevious coat may be applied by a conventional blade coater, to beimmediately followed by a lighter coat from the apparatus of the presentinvention, in order to cover prominent surface fibers, or to utilize arelatively expensive but highly advantageous coating material.

Having thus described the invention, what is claimed 1. Method forapplying liquid coating material to a moving web comprising the steps ofdirecting a single stream of high velocity air across said web in afirst distinct zone on an angle against the direction of web travel,channeling said stream of air from said first zone to a second distinctzone preceding said first zone and adjacent said web wherein said streamof air becomes substantially turbulent, depositing atomized coatingmaterial on said web in the vicinity of said second zone by injecting astream of liquid coating material into said second zone adjacent saidweb, and the immediately leveling the coating on said web by said streamof air in said first zone.

2. The method of claim 1 comprising the further step of applying a firstcoating of liquid material to said web .prior to the step of depositingatomized coating material thereon without drying the web between coats.

3. The method of claim 1 comprising the further step of drying the webafter it has passed through said first zone.

4. The method of claim 2 comprising the further step of drying the webafter it has passed through the first zone, and then calendering theweb.

5. The product made by the process of claim 4.

6. The product made by the process of claim 3.

7. Method for applying liquid coating material to a moving webcomprising the steps of supporting the moving web on a rotating roll,directing a single stream of air against the supported web, applying amist of coating material across said web with said single stream of air,and then immediately skimming the coated surface of said web with saidsingle stream of air.

1. A METHOD FOR APPLYING LIQUID COATING MATERIAL TO A MOVING WEBCOMPRISING THE STEPS OF DIRECTING A SINGLE STREAM OF HIGH VELOCITY AIRACROSS SAID WEB IN A FIRST DISTINCT ZONE ON AN ANGLE AGAINST THEDIRECTION OF WEB TRAVEL, CHANNELING SAID STREAM OF AIR FROM SAID FIRSTZONE TO A SECOND DISTINCT ZONE PRECEDING SAID FIRST ZONE AND ADJACENTSAID WEB WHEREIN SAID STREAM OF AIR BECOMES SUBSTANTIALLY TURBULENT,DEPOSITING ATOMIZED COATING MATERIAL ON SAID WEB IN THE VICINITY OF SAIDSECOND ZONE BY INJECTING A STREAM OF LIQUID COATING MATERIAL INTO SAIDSECOND ZONE ADJACENT SAID WEB, AND THE IMMEDIATELY LEVELING THE COATINGON SAID WEB BY SAID STREAM OF AIR IN SAID FIRST ZONE.
 2. The method ofclaim 1 comprising the further step of applying a first coating ofliquid material to said web prior to the step of depositing atomizedcoating material thereon without drying the web between coats.
 3. Themethod of claim 1 comprising the further step of drying the web after ithas passed through said first zone.
 4. The method of claim 2 comprisingthe further step of drying the web after it has passed through the firstzone, and then calendering the web.
 5. The product made by the processof claim
 4. 6. The product made by the process of claim
 3. 7. Method forapplying liquid coating material to a moving web comprising the steps ofsupporting the moving web on a rotating roll, directing a single streamof air against the supported web, applying a mist of coating materialacross said web with said single stream of air, and then immediatelyskimming the coated surface of said web with said single stream of air.